The invention relates to a process and associated apparatus for air fractionation by low-temperature rectification wherein the gaseous air stream to be fractionated is conducted through a cooling stage and process cold (refrigeration) is produced by compression and expansion of at least a portion of this gaseous stream. The gaseous air stream is compressed in the cooling stage and thereafter is divided into two partial streams which are at least partially cooled and subjected to engine expansion. The expansion of the first partial stream is performed at a temperature higher than that of the expansion of the second partial stream. Furthermore, both partial streams are compressed prior to expansion, utilizing the work gained during the expansion. In addition, at least a portion of each of the two partial streams is introduced to the rectification step.
Such a process has been described in U.S. Pat. No. 4,152,130. In this reference, the air to be fractionated is introduced, after precompression and prepurification wherein essentially steam and carbon dioxide are separated, into a cooling stage and utilized in the latter as the working gas. In the cooling stage, refrigeration is produced by compression and expansion of this working gas, and the refrigeration obtained is utilized in the process. Within the cooling stage, the second partial stream and a side stream of the first partial stream are subjected to engine expansion. The expansion device used to expand the side stream of the first partial stream operates at a temperature higher than that of the expansion device used to expand the second partial stream. The two expansion processes and the cooling accompanying these steps are performed in parallel to heat exchange with fractionation products in two different temperature ranges. The temperature at the outlet of the expansion device operating at a higher temperature is approximately equal to the temperature at the inlet of the expansion device operating at a lower temperature. By means of the energy obtained in the two expansion devices, both partial streams are compressed in parallel, each respectively compressed by a single compression stage.
This process is not as efficient as that of the present invention.